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A Junção PN
Site original: http://physics.slss.ie/forum
Animações: © Declan O’Keeffe
Adaptação e tradução para o português:
© Ewaldo L. M. Mehl
Isolantes
Isolantes possuem elétrons densamente ligados em sua camada de
valência, geralmente em ligações covalentes
Estes elétrons necessitam nível de energia muito elevado para tornalos elétrons de condução
Vamos aplicar uma diferença de potencial sobre o material isolante
acima…
A energia fornecida a cada elétron não é suficiente para romper as
ligações covalentes
Isolantes possuem portanto alta resistividade
Slide 2
Condutores
Condutores tem elétrons fracamente ligados aos átomos em sua
camada de valência
Estes elétrons requerem pouca quantidade de energia para torna-los
elétrons de condução
Valos aplicar uma DDP através do condutor…
A força sobre cada elétron é suficiente para liberta-lo da sua órbita e
assim os elétrons podem “pular” para outros átomos: existe portanto
condução da corrente elétrica
Condutores possuem portanto baixa resistividade
Slide 3
Semicondutores
Semicondutores apresentam resistividade intermediária
entre a dos condutores e isolantes
Os elétrons de um semicondutor participam de ligações
covalentes e por isso, à princípio, não estariam disponíveis
para a condução da corrente elétrica; no entanto, um nível
de energia relativamente BAIXO já é capaz de colocar
alguns elétrons em estado de condução
Este fenômeno era já conhecido para o silício e para o
germânio
Slide 4
Silício (Si)
O Siício tem valência 4,
i.e. 4 elétrons na camada
mais externa
Esta figura mostra
apenas os elétrons
compartilhados
Cada átomo de silício
compartilha seus 4
elétrons de valência com
4 átomos vizinhos
Nesta representação os
elétrons compartilhados
são mostrados como
ligações covalentes na
forma de linhas
horizontais e linhas
verticais ligando os
átomos
Slide 5
Silício: rede cristalina (simplificada)
Agora estendemos o
arranjo através de um
pedaço de cristal de
silício…
Obtemos uma
REPRESENTAÇÃO
SIMPLIFICADA de um
cristal de silício
Esta seria a situação do
cristal na temperatura de
0K
Não há elétrons livres : o cristal não conduz corrente elétrica e portanto se
comporta como se fosse um MATERIAL SOLANTE
Slide 6
Electron Movement in Silicon
However, if we apply
a little heat to the
silicon….
An electron may gain
enough energy to
break free of its
bond…
It is then available
for conduction and is
free to travel
throughout the
material
Slide 7
Hole Movement in Silicon
Let’s take a closer
look at what the
electron has left
behind
There is a gap in the
bond – what we call
a hole
Let’s give it a little
more character…
Slide 8
Hole Movement in Silicon
This hole can also
move…
An electron – in a
nearby bond – may
jump into this hole…
Effectively causing
the hole to move…
Like this…
Slide 9
Heating Silicon
We have seen that,
in silicon, heat
releases electrons
from their bonds…
This creates
electron-hole pairs
which are then
available for
conduction
Slide 10
Intrinsic Conduction
Take a piece of
silicon…
And apply a potential
difference across it…
This sets up an
electric field
throughout the
silicon – seen here as
dashed lines
When heat is applied an electron is
released and…
Slide 11
Intrinsic Conduction
The electron feels a
force and moves in
the electric field
It is attracted to the
positive electrode
and re-emitted by the
negative electrode
Slide 12
Intrinsic Conduction
Now, let’s apply
some more heat…
Another electron
breaks free…
And moves in the
electric field.
We now have a
greater current than
before…
And the silicon has
less resistance…
Slide 13
Intrinsic Conduction
If more heat is
applies the process
continues…
More heat…
More current…
Less resistance…
The silicon is acting
as a thermistor
Its resistance decreases
with temperature
Slide 14
The Thermistor
The thermistor is a heat sensitive
resistor
 When cold it behaves as an
insulator i.e. it has a very high
resistance
 When heated, electron hole pairs
are released and are then available
for conduction as has been
described – thus its resistance is
reduced

Thermistor
Symbol
Slide 15
The Thermistor

Thermistors are used to measure
temperature

They are used to turn devices on,
or off, as temperature changes

They are also used in fire-warning
or frost-warning circuits
Thermistor
Symbol
Slide 16
The Light Dependent Resistor (LDR)
The LDR is very similar to the
thermistor – but uses light energy
instead of heat energy
 When dark its resistance is high
 As light falls on it, the energy
releases electron-hole pairs
 They are then free for conduction
LDR Symbol
 Thus, its resistance is reduced

Slide 17
The Light Dependent Resistor (LDR)

LDR’s are used as light meters

LDR’s are also used to control
automatic lighting

LDR’s are used where light is
needed to control a circuit – e.g.
Light operated burgler alarm
LDR Symbol
Slide 18
The Phosphorus Atom
Phosphorus is
number 15 in the
periodic table
It has 15 protons and
15 electrons – 5 of
these electrons are in
its outer shell
Slide 19
Doping – Making n-type Silicon
Relying on heat or
light for conduction
does not make for
reliable electronics
Suppose we remove
a silicon atom from
the crystal lattice…
and replace it with a
phosphorus atom
We now have an electron that is not bonded – it is thus free for
conduction
Slide 20
Doping – Making n-type Silicon
Let’s remove another
silicon atom…
and replace it with a
phosphorus atom
As more electrons
are available for
conduction we have
increased the
conductivity of the
material
Phosphorus is called
the dopant
If we now apply a potential difference
across the silicon…
Slide 21
Extrinsic Conduction – n-type Silicon
A current will
flow
Note:
The negative
electrons move
towards the
positive
terminal
Slide 22
N-type Silicon




From now
on n-type
will be
shown like
this.
This type of silicon is called n-type
This is because the majority charge carriers are
negative electrons
A small number of minority charge carriers – holes –
will exist due to electrons-hole pairs being created in
the silicon atoms due to heat
The silicon is still electrically neutral as the number of
protons is equal to the number of electrons
Slide 23
The Boron Atom
Boron is number 5
in the periodic table
It has 5 protons and
5 electrons – 3 of
these electrons are
in its outer shell
Slide 24
Doping – Making p-type Silicon
As before, we
remove a silicon
atom from the crystal
lattice…
This time we replace
it with a boron atom
Notice we have a
hole in a bond – this
hole is thus free for
conduction
Slide 25
Doping – Making p-type Silicon
Let’s remove another
silicon atom…
and replace it with
another boron atom
As more holes are
available for
conduction we have
increased the
conductivity of the
material
Boron is the dopant
in this case
If we now apply a potential difference
across the silicon…
Slide 26
Extrinsic Conduction – p-type silicon
A current will
flow – this time
carried by
positive holes
Note:
The positive
holes move
towards the
negative terminal
Slide 27
P-type Silicon




From now
on p-type
will be
shown like
this.
This type of silicon is called p-type
This is because the majority charge carriers are positive
holes
A small number of minority charge carriers – electrons –
will exist due to electrons-hole pairs being created in the
silicon atoms due to heat
The silicon is still electrically neutral as the number of
protons is equal to the number of electrons
Slide 28
The p-n Junction
Suppose we join a piece of p-type silicon to a piece
of n-type silicon
We get what is called a p-n junction
Remember – both pieces are electrically neutral
Slide 29
The p-n Junction
When initially joined
electrons from the
n-type migrate into the
p-type – less electron
density there
When an electron
fills a hole – both the
electron and hole
disappear as the gap
in the bond is filled
This leaves a region with no free charge carriers – the depletion
layer – this layer acts as an insulator
Slide 30
The p-n Junction
0.6 V
As the p-type has
gained electrons – it
is left with an overall
negative charge…
As the n-type has
lost electrons – it is
left with an overall
positive charge…
Therefore there is a voltage across the junction – the junction
voltage – for silicon this is approximately 0.6 V
Slide 31
The Reverse Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type negative
n-type positive
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be reverse – biased
Slide 32
The Reverse Biased P-N Junction
Negative electrons
in the n-type feel
an attractive force
which pulls them
away from the
depletion layer
Positive holes in
the p-type also
experience an
attractive force
which pulls them
away from the
depletion layer
Thus, the depletion layer ( INSULATOR ) is
widened and no current flows through the
p-n junction
Slide 33
The Forward Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type postitive
n-type negative
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be
forward – biased
Slide 34
The Forward Biased P-N Junction
Negative electrons
in the n-type feel a
repulsive force
which pushes
them into the
depletion layer
Positive holes in
the p-type also
experience a
repulsive force
which pushes them
into the depletion
layer
Therefore, the depletion layer is eliminated
and a current flows through the p-n junction
Slide 35
The Forward Biased P-N Junction
At the junction
electrons fill holes
Both disappear
as they are no
longer free for
conduction
They are
replenished by the
external cell and
current flows
This continues as long as the external voltage
is greater than the junction voltage i.e. 0.6 V
Slide 36
The Forward Biased P-N Junction
If we apply a
higher voltage…
The electrons feel
a greater force
and move faster
The current will
be greater and
will look like
this….
The p-n junction is called a DIODE
and is represented by the symbol…
The arrow shows the
direction in which it
conducts current
Slide 37
The Semiconductor Diode
The semiconductor diode is a p-n
junction
 In reverse bias it does not conduct

In forward bias it conducts as long
as the external voltage is greater
than the junction voltage
 A diode should always have a
protective resistor in series as it
can be damaged by a large current

Slide 38
The Semiconductor Diode




The silver line drawn on one side of the
diode represents the line in its symbol
This side should be connected to the
negative terminal for the diode to be
forward biased
Diodes are used to change alternating
current to direct current
Diodes are also used to prevent damage in
a circuit by connecting a battery or power
supply the wrong way around
Slide 39
The Light Emitting Diode (LED)





Some diodes emit light as they conduct
These are called LED’s and come in various colours
LED’s have one leg longer than the other
The longer leg should be connected to the positive
terminal for the LED to be forward biased
LED’s are often used as power indicators on radios,
TV’s and other electronic devices
Symbol
Slide 40
The Characteristic Curve of a Diode





Diodes do not obey Ohm’s Law
A graph of CURRENT vs
VOLTAGE for a diode will not
be a straight line through the
origin
The curve will look like this one
Note how the current increases
dramatically once the voltage
reaches a value of 0.6 V approx.
i.e. the junction voltage
This curve is known as the
characteristic curve of the diode
Slide 41
Slide 42